The cause of death was not immediately known, and Wu's son Harrison and former wife China Lee were travelling to the Central American nation to bring home his remains, Ms Noonan said.

"He was a real hero," Ms Noonan said by phone from New York. "Harry's work will continue, it will not stop."

Wu was born into a prosperous family in Shanghai who saw most of their property confiscated following the civil war victory of Mao Zedong's communists in 1949. He studied geology at university but fell foul of the authorities for his criticism of the Soviet Union, China's then-ally, and was sentenced in 1960 at the age of 23 to China's prison camp system known as laogai, or "reform through labour".

Laogai was notorious for punishing intellectuals and political prisoners with long sentences and brutal conditions and the camps were blamed by some for causing millions of deaths. According to his autobiography, Wu spent various terms in 12 different camps, experiencing harsh work regimens on farms, coal mines and work sites, along with beatings, torture and near starvation.

Released in 1979 following Mao's death three years earlier, Wu moved to the United States in 1985. He taught, wrote and founded the Laogai Research Foundation while returning frequently to China to carry out research on the labour camp system.

Having become a US citizen, Wu was arrested during a visit to China in 1995 and sentenced to 15 years on espionage charges. He was immediately deported to the US where he continued his work documenting Chinese human rights abuses and was a frequent speaker before Congress and at academic events.

The Washington DC-based foundation established the Laogai Museum in 2008 to "preserve the memory of the laogai's many victims and serve to educate the public about the atrocities committed by China's communist regime", according to the foundation's website.

China has since formally eliminated laogai along with a milder version known as laojiao, or "reform through education", although penal labour remains a key feature of the Chinese prison system.

Wu was the author of books about his prison experience and later advocacy, including The Chinese Gulag, Bitter Winds and Troublemaker.

A Catholic, he campaigned for other causes, including international labour rights, religious freedom, and an end to the death penalty, forced organ harvesting and China's often coercive population control policies.

He was a strong backer of other political prisoners and critics denounced by Beijing, including exiled Tibetan spiritual leader the Dalai Lama and 2010 Nobel Peace Prize laureate Liu Xiaobo, who is serving a prison sentence in China for advocating political reforms.